Voltage sags and swells can occur on utility grids and local grids. A voltage sag is a short duration decrease in voltage (e.g., root mean square (RMS) voltage). Common causes of voltage sags are motors starting, customer load additions, and large load additions in a utility service area. For example, an inductive load such as a transformer or motor can cause large sags in grid voltage when starting. A voltage swell is a short duration increase in voltage values. A voltage swell can be caused by removing a large load or by switching in a capacitor bank that is too large for the prevailing conditions.
Such voltage sags and swells can be accompanied by reactive power flow. For example, inductive loads such as transformers and motors absorb reactive power, and capacitive loads such as capacitor banks or long cables generate reactive power. However, local grids may not have the ability to compensate for such reactive power flow in a real-time fashion.